Explicitly sort contents of directory

os.listdir depends on the ordering of the filesystem,
and not all filesystems list the directory contents in
alphabetical order.

To test this, I commited just the test and watched it fail on travis: https://travis-ci.org/jamesls/botocore/jobs/8957523

Then I rebased in the fix and watched it pass on travis: https://travis-ci.org/jamesls/botocore/builds/8957679